Fix integer overflow bug

[phstratmann]

Issue Number: #126

Objective of pull request:
If the total cost of a current solution is too large, then the CostIntegrator currently shows an integer overflow. The resulting cost is thus very negative, and thus the solver stops and claims that it found a particularly good solution.
This PR introduces int32 cost instead of the previous int24 cost.
There is a corresponding PR in Lava-on-Loihi https://github.com/intel-collab/frameworks.ai.lava.lava-loihi/pull/301.

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• Issue created that explains the change and why it's needed
• Tests are part of the PR (for bug fixes / features)
• Docs reviewed and added / updated if needed (for bug fixes / features)
• PR conforms to Coding Conventions
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• Bugfix

[GaboFGuerra]

Thanks Philipp, that was a very larger refactoring, great job!
I left a few comments but set the PR as approved to speed up integration as nothing is really critical, please go through the comments and address at your discretion.

Thanks :)